I would vote to head out to Windsor. Or just relax in T5B. When you add in getting off the plane, security, wandering to and from the gates, and boarding, plus any pre teen distractions your 5 hour layover could quickly become closer to three. There are quite a few preteen friendly shops depending on taste and eating selections that they may find more interesting than sitting in a lounge…

after having done a relatively non stressful T5 to T3 connection where it all actually worked well in terms of busses arriving, queues etc, no mater how good I may think the CX, CL, QF lounges in T3 are, there is no way I would spend time fluffing around needlessly.